I have MikkoEevert Photo of the Day Instagram page(ME_PotD). I give there one photo challenge per day. Once or twice a week I publish those challenges here in my web page. With some tips and pep talks. There is no time limit to participate. Start when ever you like. No stress just have fun and perhaps learn something from photography.
You can do these challenges with what ever camera you normally use. Publish your photo in Instagram and tag #ME_PotD with challenge number.